1. Technical Field
The present disclosure relates to fuel control devices, and in particular, to fuel manifolds and valves for turbines.
2. Discussion of the Art
Turbine engines have a set of rotating turbine blades that compress air into the combustion area where fuel is injected and ignited. Fuel is delivered through metering orifices to burners in the combustion chamber under pressure through a fuel line. Combustion of the fuel turns a downstream set of blades from which energy is extracted and which can also be used to drive the compressor blades. The combustion area of the turbine can be divided by and contained in combustion cans. There are anywhere from six to eighteen such cans in modern turbine systems used to generate power. Each can has a burner to ignite the fuel as well as injectors for injecting the fuel into the can for combustion. Typically, some of the injectors are designated as primary injectors and one or more are secondary injectors. The primary injectors are used during the start-up sequence and at periods of lower fuel consumption, but otherwise both the primary and secondary injectors are used during normal operation of the turbine, each with several burner nozzles that ignite the fuel at light-off and sustain combustion during operation. A large manifold is ordinarily used to contain and route fuel to the various injectors of the combustion cans.
The combustion cans of the turbine are high pressure and temperature environments. It is typical for the environment surrounding the combustion cans to reach temperatures of 400° F., and for the combustion chamber temperature to near 2,000° F. The liquid fuel is consumed at a rate of about 20 gallons per minute at a high fuel pressure of about 1200 psig. This extreme environment is very hard on the fuel control components of the turbine fuel system, particularly for dual fuel turbines in which during sustained gaseous burn, the liquid fuel system remains inoperable for long periods of time.
A principal concern is the formation of the coke, or the tarry deposits left after the distillate or volatile components of the fuel are driven off by heat, on the metering orifices and other working surfaces of the liquid fuel control components. Coke deposits arise primarily from the presence of residual fuel left in the convoluted passages and dead spaces present in fuel atomizer, burner nozzles, control valves, fuel manifolds and other components subjected to the high heat of combustion. Residual liquid fuel left in the liquid fuel control components during gaseous operation will begin to coke at temperatures of about 250-280° F. in the presence of oxygen, which are well under the combustion temperature.
To reduce the effects of coking liquid fuel can be circulated through a heat exchanger to cool the temperature of the liquid fuel distillate to below the coking threshold temperature during operation of the turbine in gaseous fuel mode, see U.S. Pat. No. 6,729,135. However, this system requires a heat exchanger and either a separate fuel recirculation pump or increased duty on the main fuel pump. Moreover, because the recirculation lines carry liquid fuel, these lines, along with any recirculation control components, present yet another location for coking to occur when the recirculation system is not operating. To avoid this, during liquid fuel operation some of the liquid fuel must be made to bypass the combustor to flow through the recirculation system.
Dedicated cooling circuits can be used which avoid the aforementioned problems with using fuel as the coolant. However, this too can be problematic if not properly monitored and controlled. For one thing, if the coolant runs too hot, then adequate cooling may not be achieved such that components may undergo the coking problem discussed above. Also, if the coolant were to leak, otherwise flow at an insufficient rate, not only could coking occur, but in the event of a leak in which coolant is sprayed onto the turbine, could result in “turbine rub”, a condition in which housing components of the turbine shrink or contract from cooling caused by the leak and interfere with the turbine blades. If left unchecked, turbine rub can cause significant damage to the large, rotating turbine blades and render the turbine inoperable. Further, if the coolant were to run too cold, for example if at an excessive flow rate or supply conditions were at an insufficient temperature, then the coolant could actually reduce the fuel temperature sufficiently to cause “waxing”, a condition in which the fuel media, such as diesel fuel, begins to turn into a paraffin material. This condition disrupts the fuel delivery system and can similarly render the turbine inoperable. In addition to the waxing problem, excessively cold coolant can interfere with the proper operation of other components or sub-systems, for example, fogging of optical pyrometer.
Alternatively, the liquid fuel line components can be purged with air, water or other suitable media. However, in order to permit purging of the liquid fuel line a purging line must join with the fuel line. It is important that the fueling and purging operations be isolated so that fuel does not go down the purge line and hot gases do not travel up the fuel line to the fuel supply. A three-way purge valve for this purpose is disclosed in U.S. Pat. No. 6,050,081. Such a purge valve is significantly more reliable than simple check valves for preventing backwash and is more resistant to coking. As disclosed, a spool valve having an enlarged middle section was used to shuttle between positions alternatively blocking the combustion can from either the purge air line or the fuel line. The spool is biased to close off the fuel line and is urged to open the fuel line by a pilot air actuated piston. Thus, when fuel is to be closed off from the engine, the spool valve will return to its initial position thereby allowing the burner nozzles and the downstream side of the spool to be purged to reduce or eliminate coking in these areas.
To avoid the need to use dedicated purge valves for each injector, a separate distributor can be mounted to each combustion can to act as a manifold to which the several fuel lines connect before the fuel is routed to the individual injectors. This additional component and additional lines add significant cost, assembly and size to the system. And, these parts create additional areas for coking to occur, particularly given that the distributor is typically mounted directly to the combustion can which realizes extreme temperatures during combustion. A distributor three-way purge valve is disclosed in U.S. Pat. No. 6,931,831 that addresses the aforementioned problem by combining the functionality of the three-way purge valve with that of a distributor in a single unit.
While these valves provided a significant improvement to turbine fuel systems, such distributor valves are not designed to work with conventional fuel manifolds. Thus, coking and cross-contamination of fuel and purge media remain concerns for conventional fuel manifolds.